Self cleaning windows and titanium dioxide

Since
self cleaning windows have been around since 2001, I suppose the main
characteristics can be probably classified as common knowledge by now.

For
the sake of thoroughness, let me quickly run through them. Not a very difficult
task since there really are only two of them. A window with self cleaning properties is defined
by having the glass surface treated so it displays one of the two key properties:
hydrophobicity or hydrophilicity.

Hydrophobicity,
literally meaning “fearful of water”, is achieved by modifying the architecture
of the usual smooth surface of the glass with the objective to create “little
bumps”. A material gets wet because water easily adheres to it, transforming
from droplets into a thin layer. The smoother the surface, the wetter it gets.
As the surface gets “bumpy” the droplets keep their shape and roll off the
surface instead of sticking to it. The effect was discovered and shaped after
examples found in nature and it is still called “the lotus effect” or sometimes
“the butterfly wing effect”. In self cleaning windows the modifications of
glass surface are made at nano structure level so as to not alter the
transparency properties.

Hidrophilicity
(“love of water”) is the exact opposite. The glass surface is treated as to
attract water. And then titanium dioxide’s photo catalytic effect comes into
play. It is also known as “the Honda-Fujishima effect” based on the first
researcher that documented it. Basically, titanium dioxide has the unique
ability to decompose water and certain other organic materials when exposed to
light (more precisely, to UV rays).

Thus,
hydrophilic auto cleaning windows are made by apply a thin titanium dioxide
layer on the surface of the glass. Water droplets containing impurities that
come in contact with it are decomposed by the photo catalytic effect, thus
keeping the surface clean. See an example in the picture

Quick fact: Titanium dioxide treated glass requires
a period of 5 to 7 days to “activate” the photo catalytic effect. The oxidizing
(decomposition) capability is stronger than chlorine bleach.

All
this sounds pretty exciting and innovative, a marvel of the modern era. But
there is a long way from the exciting range of a theoretical discovery and the
practical application. The first thing that is to be noted is that both of
these solutions require water in order to function properly. If you’re living
in a particular dry climate or encounter a particularly longer dry spell, you
may find yourself needing professional cleaning services again.

This
may not be such a disaster, but self cleaning windows are priced accordingly
with the high-tech profile, and installation costs are 10 to 20% higher than for
normal windows. Also, special care is required during installation and use.
Silicon based sealants are specifically excluded by virtually all the
manufactures as silicone is known to react with the titanium dioxide layer and
destroy it. The layer can also be destroyed by careless mechanical operation
such as relocating and maintenance operations.

There
are some substances that simply are not affected by the photo catalytic effect –
such as blood, stucco or opaque paints. Sometimes, even a simple, common
residue as bird muck, if dried up, has to be removed manually.

Quick
fact: Most of the formerly known self cleaning windows are now being remarketed
as “low maintenance windows”.

I
personally think that the research funds that went into developing self cleaning
windows would have been much useful if employed in other phocatalytic related
fields – such as clean energy generators, hydrogen generation, not to mention
the medical field for innovative cures involving photo catalytic destruction of
viruses or tumors.